The present invention relates to low offset demodulators, and more particularly to a low offset position demodulator for use in a positioning servo of a disk drive, or similar device.
It is common practice in the electromechanical arts to use a position servo to control the positioning of a first movable element with respect to a second, usually stationary, element. Such controlled motion is typically realized through the use of a suitable position transducer that senses the relative location of the first element with respect to the second element. The relative location is typically indicated by an electronic signal, derived from or generated by the transducer, that assumes a prescribed value, such as a null point, when the desired position is sensed; and assumes a value other than the prescribed value when the desired location is not sensed. The electronic signal thus derived from the transducer, typically referred to as a "position signal", is then used as the error signal for a positioning servo system. The position servo, in response to the position signal, causes the first element to move relevant to the second element until the position signal is driven to its prescribed level, indicating that the desired position has been obtained. Any errors in the position signal, such as might be caused by dc offset, translate directly to positioning errors between the first and second elements. Thus, such errors limit the accuracy of the positioning servo.
In the data processing industry, for example, it is common practice to store large quantities of data in digital form upon rotating disk storage media. In order to access a desired location, or data track, on the disk, a read/write head must be positioned radially with respect to the disk so that the desired track can be accessed. Once positioned, data may be written to or read from the tracks located on the disk. The amount of data that may be written to or read from a given disk--the data density--is in large part a function of how closely the data tracks may be placed together. The achievable track density is, in turn, largely a function of the accuracy of the positioning servo used to position the read/write head. It is thus desireable to utilize a position transducer in such disk storage schemes that generates a position signal (or that generates a signal from which a position signal may be derived) wherein a minimum amount of dc offset is present.
There are many types of disk storage systems. One type of popular disk storage system employs a plurality of rigid magnetic disks that are stacked one on top of the other on a common shaft and rotated at a steady speed. Magnetic read/write heads, one for each side of each disk, are moved in and out radially with respect to the disk in response to machine commands. All the magnetic heads are mounted on a single head drive means and move together until the head is positioned at the distance from the center of the disk where the particular record sought to be read or written is located. As explained earlier, it is desireable to provide a magnetic storage media with as much data storage capacity as possible. To this end, it is desireable to locate the tracks as close to one another as possible. This requires that dc offset present in the associated positioning signal must be eliminated or greatly minimized.
In order to define the location of data tracks on the disk to which data may be written or from which data may be read, it is common practice in such a common spindle, multiple disk drive to dedicate one surface of one of the disks to the storage of permanently encoded position information. Such position information is prewritten onto the disk, using specially designed servo writing equipment, in a plurality of servo tracks. A servo read head, tied to all the other read heads, then reads the position information as the disk pack rotates. It is from this information that the above-described position signal is derived. The positioning servo may then selectively and repeatidly lock onto and follow a given servo track or tracks are required. The servo tracks located on the servo disk thus define the location where data tracks will exist on the surface of the other disks.
In order to optimize the transfer of information from the disk surface to the read head, it is usually advantageous to employ a carrier signal that is modulated with the position information. This carrier signal must then be demodulated in order to recover the position information and convert it to an appropriate position signal for use by the position servo. Unfortunately, this process of demodulation and conversion creates numerous avenues through which undesirable dc offset may be introduced into the position signal. What is needed, therefore, is a means to demodulate a servo carrier signal without allowing any such offset to be introduced into the resulting position signal.
Numerous demodulation circuits and demodulating schemes are known in the art. U.S. Pat. Nos. 3,927,378; 3,688,205; 4,250,458; and 4,286,224 are representative. However, none of these references, or any other references known to the inventors, address the problem of reducing the dc offset of a demodulator circuit used in a track-following servo as does the present invention. Nor do any known references show or suggest the particular circuit scheme used by the present invention to achieve its low offset. Further, while prior art is known that relates to track-following servos, such as U.S. Pat. Nos. 3,691,543 and 4,115,823; as well as IBM Technical Disclosure Bulletin, Vol. 21, No. 2, pp. 804-05 (July 1978); and IBM Technical Disclosure Bulletin, Vol. 22, No. 12, pp. 5436-38 (May 1980), this art really does not address the problem of reducing demodulator offset as does the present invention. Rather, the thrust of such art is to disclose a particular encoding scheme that could be used to place the position information on the desired servo tracks. The present invention, in contrast, may be applicable to any type of encoding scheme or demodulation technique (e.g., area detection or peak detection) that may be used.
The problem of demodulator offset has long plagued the industry. The only effective prior art solutions known to the inventors for eliminating or reducing such demodulator offset is to provide manual adjustment means to trim the offset out once the circuit has been built. Alternatively, components may be carefully and manually selected to find matching characteristics. Further, it is known in the art to use a very large number of expensive components, specified for their precise values in the fabrication of such circuits. All of these techniques are undesirable because they are expensive as well as error pron. What is needed, therefore, is a method and technique for eliminating dc offset in demodulator circuits that is both inexpensive and easy to implement.